Gold, Silver, and Bronze are just regular "tiers" for players, they're just an arbitrary rating thing. They cost differently when you buy contracts for them and come with different amount of contracts when you first get them. Golds have 3 sig. skill slots, silvers 2, bronze 1. Other than that, they're just normal players.

Sapphires are slightly better versions of their gold (I think they're all gold) counter-parts and can be obtained from road to playoffs, tournaments, and domination mode

Diamonds are vastly better versions of their gold counter-parts and can be unlocked from codes that 2k never gives away. They also have 5 sig. slots as opposed to 3. (Although Chris Smoove has said that there's a rumor that only 3 of em works in game. Not sure about that.)

There's a ruby curry that's barely better than the gold curry

Emeralds are a "season update" kind of thing, they're 2k's way of not updating the players directly, instead choosing to release another pack. These guys are the same as gold players in terms of their contracts and sig slots I believe.

Team packs are generally not great, since they only give you team items, which suck. Gold packs are expensive. Best bet is indeed historic packs like the other guys said. They give you a historic player and sig. skills that you can equip. Many of the best legends require you to complete the team to unlock, which is hard when it's just a mixed jumbo pack now, but some don't, e.g. 96 Jordan, Jerry West, Wilt etc. If you're lucky, you might pull one of these guys

Emerald is definitely the best value. 12k VC, a sig skill, a +5 contract, and a chance for gold players? Definitely worth it. Especially so given the players aren't that rare. I got LMA, Ellis, and Plumlee in like 3 tries, not to mention some non emerald players.

I was recently in the same boat as you. Emerald players are merely the "biggest movers. " these are the people whose stats have changed a lot during the season so these are those updated stats. Still not sure what the difference between sapphire and Diamond is but I think they are just upgraded/changed from the gold players.

The emerald packs are great for the beginning because they reward with, usually, pretty decent players and they aren't too much. Historic packs are also pretty good.

The Ruby, Sapphire, and Diamond players are just better versions of the cards. In other words, the stats are bettered in the different versions. For example, Gold Steph Curry and Ruby Steph Curry have different 3 point ratings, which makes Ruby Steph Curry better. Diamond players are the cream of the crop, and are super rare. Ruby players are a step up from Gold, then Sapphire, and finally Diamond are the best players.

Others have already answered the types of players so I'll tell you how to get a good team: Domination.

Get an all bronze team. Don't use anyone who isn't bronze. Get some solid three point shooters, drive and kick out for the assist. If you get 6000 points (which is easiest to do with bronze players) you are guaranteed at least a silver player from the team you're up against, and you have a solid chance at a gold (I've gotten Lebron, Griffin, Lee, and Cousins).